This has been argued both ways, with lots of anecdotal evidence that dressing nicely gets you better treatment and as much that it doesn't.

My experience has been that it doesn't matter. I've gotten operational upgrades when I was incredibly scruffy after a full day of hiking around Chile, and when I checked in at an in-town counter much earlier in the day and they couldn't possibly know what I might look like several hours later. I generally travel as comfortably as possible, which means very casual (though usually not sloppy) clothing, and never felt it made any difference. YMMV, of course.

As Kevin, an AA purser with whom I flew ORD-LHR in F a couple of years ago and again NRT-ORD in J last month, put it: "Once you're in here, there's only one kind of service: the best we can do." He was specifically answering a question about differences in how paid F, upgrades and non-revs are treated, but I think it would apply to dress (within reason) as well.